1. Field of the Invention
The present invention relates to a device for insulating vibrations and supporting a vibration generating means such as an engine of a vehicle, and more particularly to a so-called fluid sealed type vibration insulating device which is arranged to sufficiently decrease a large magnitude vibration input without providing a large and expensive device.
2. Description of the Prior Art
Generally, an engine mount functions as a vibration insulating device in use for supporting a power unit of a vehicle and is mainly required to perform a vibration insulating function relative to vibrations due to idling, booming noises and noises during a vehicle acceleration. However, in order to reduce such idling vibrations, which has a relatively large magnitude ranging from about 20 to 30 Hz, the vibration insulating support device is required to have characteristics of a high dynamic spring constant and a high damping. In contrast, in order to reduce booming (echo sound) vibration and/or acceleration vibrations, which are of a vibration of a relatively small magnitude ranging from about 80 to 800 Hz, the vibration insulating device is required to have characteristics of a low dynamic spring constant and low damping. Accordingly, it is difficult to insulate all vibrations by a normal elastic engine mount or by an engine mount of a conventional fluid sealed type.
A fluid sealed type vibration insulating device, which can generate active supporting force has been proposed so as to adapt to the above mentioned requirements. A Japanese Patent Provisional Publication No. 3-24338 discloses such a fluid sealed type vibration insulating device which is provided with a fluid chamber defined by a supporting elastomer and a moveable plate. The fluid chamber is filled with fluid and is changed in volume by displacing the movable plate through an electromagnetic actuator so as to generate a control force for canceling force transmitted to the vibration insulating device.
However, since such conventional vibration insulating devices have been arranged such that the movable plate is elastically supported through a ring-shaped plate spring, various limitations have been applied to the conventional vibration insulating device. For example, since such a ring-shaped plate spring tends to be high in spring rigidity per unit thickness, an elastically deformable range of the plate spring becomes narrow so as to limit a displacement amount of the movable plate in proportion to a controlled displacement. Also, if the plate spring is formed by a thin plate in order to broaden a proportionally deformable range of the movable plate, durability of the plate spring tends to be degraded.